1. Field of the Invention
The present invention relates to power and wavelength control for semiconductor diode lasers, and particularly, power and wavelength control for Sampled Grating Distributed Bragg Reflector (SGDBR) semiconductor lasers.
2. Description of the Related Art
Diode lasers are being used in such applications as optical communications, sensors and computer systems. In such applications, it is very useful to employ lasers that can be easily adjusted to output frequencies across a wide wavelength range. A diode laser which can be operated at selectably variable frequencies covering a wide wavelength range, i.e. a widely tunable laser, is an invaluable tool. The number of separate channels that can utilize a given wavelength range is exceedingly limited without such a laser. Accordingly, the number of individual communications paths that can exist simultaneously in a system employing such range-limited lasers is similarly very limited. Thus, while diode lasers have provided solutions to many problems in communications, sensors and computer system designs, they have not fulfilled their potential based on the available bandwidth afforded by light-based systems. It is important that the number of channels be increased in order for optical systems to be realized for many future applications.
For a variety of applications, it is necessary to have tunable single-frequency diode lasers which can select any of a wide range of wavelengths. Such applications include sources and local oscillators in coherent lightwave communications systems, sources for other multi-channel lightwave communication systems, and sources for use in frequency modulated sensor systems. Continuous tunability is usually needed over some range of wavelengths. Continuous tuning is important for wavelength locking or stabilization with respect to some other reference, and it is desirable in certain frequency shift keying modulation schemes.
In addition, widely tunable semiconductor lasers, such as the sampled-grating distributed-Bragg-reflector (SGDBR) laser, the grating-coupled sampled-reflector (GCSR) laser, and vertical-cavity spontaneous emission lasers which micro-electro-mechanical moveable mirrors (VCSEL-MEMs) generally must compromise their output power in order to achieve a large tuning range. The basic function and structure of SGDBR lasers is detailed in U.S. Pat. No. 4,896,325, issued Jan. 23, 1990, to Larry A. Coldren, and entitled xe2x80x9cMULTI-SECTION TUNABLE LASER WITH DIFFERING MULTI-ELEMENT MIRRORSxe2x80x9d, which patent is incorporated by reference herein. Designs that can provide over 40 nm of tuning range have not been able to provide much more than a couple of milliwatts of power out at the extrema of their tuning spectrum. However, current and future optical fiber communication systems as well as spectroscopic applications require output powers in excess of 10 mW over the full tuning band. Current International Telecommunication Union (ITU) bands are about 40 nm wide near 1.55 xcexcm, and it is desired to have a single component that can cover at least this optical bandwidth. Systems that are to operate at higher bit rates will require more than 20 mW over the full ITU bands. Such powers are available from distributed feedback (DFB) lasers, but these can only be tuned by a couple to nanometers by adjusting their temperature. Thus, it is very desirable to have a source with both wide tuning range ( greater than 40 nm) and higher power ( greater than 20 mW) without a significant increase in fabrication complexity over existing widely tunable designs. Furthermore, in addition to control of the output wavelength, control of the optical power output for a tunable laser is an equally important endeavor as optical power determines the potential range for the laser.
Thus, there is a need in the art for devices and methods to precisely control of the power and wavelength output of semiconductor lasers. The present invention meets these needs.
The present invention discloses devices and methods for controlling the power and wavelength of semiconductor lasers. A typical optical output power and output wavelength control systems of the invention for use with a sampled grating distributed Bragg reflector (SGDBR) laser comprises a controller for providing current or voltage inputs to the laser and current or voltage inputs to the thermal electric cooler controlling the optical output power and output wavelength and an external reference receiving an optical output from the laser and providing a reference output to the controller, wherein the controller compares the optical output power and output wavelength of the laser to the reference output and locks the optical output power and output wavelength of the laser to the external reference.
To accurately control the optical output power and precisely control the output wavelength of a tunable laser, a feedback loop is used in conjunction with an external reference wavelength locker, e.g., a Fabry-Perot Etalon reference though not limited to, to lock the SGDBR laser optical output power and wavelength to the reference. The feedback loop compensates for the drift of the controller current sources, as well as providing compensation for long-term degradation of the SGDBR laser. Further, the present invention provides compensation for the SGDBR laser operating points over an ambient temperature range.
The power and wavelength controls may operate as independent controls of the SGDBR laser, or can be controlled in an interdependent manner to accurately provide a given optical power and output wavelength regardless of the length of time that the SGDBR laser has been in use, the ambient temperature, or other external conditions of where the SGDBR laser is operating.
In an independent control system, each control algorithm induces changes in one current or operating temperature independent of the other using proportional integral control routines. In an interdependent control schema, the algorithm induces primary changes in one current or operating temperature and corrects for secondary changes in the other currents with an adaptive filter or estimator. This approach compensates for wavelength shifts or power changes and mirror misalignment induced when the control adjusts its primary variable. These changes are then used to compensate values in the aging model for the other wavelength settings.